The roles of protein tyrosine phosphatases in immune cell function are being studied. Of particular interest are phosphatases that are involved in the cellular signaling cascade initiated at the immune response receptors. In initial work, protein tyrosine phosphatases have been identified in mast cells and basophils by biochemical and molecular cloning methods. A phosphatase activity associated with the high affinity IgE receptor in mast cells has been found. It preferentially dephosphorylates the receptor subunits but not other key signaling proteins. Therefore, it may be an important regulator of IgE receptor function. Another protein tyrosine phosphatase that seems to be restricted in its distribution to hematopoietic cells has been cloned and sequenced. It localizes to discrete subcellular compartments and becomes tyrosine phosphorylated upon IgE receptor aggregation. Thus, it too is likely to be involved in the signaling process initiated at immune response receptors.